This invention relates to high efficiency x-ray lenses, particularly to a sputtered-sliced technique for fabrication of transmissive optics for x-rays, and more particular to a process for fabricating gradient index x-ray optics in the 0.5 to 4.0 keV energy region.
Prior to setting forth the background technology developments which have led to the present invention, the following definitions of terms used hereinafter is set forth, with synonyms for the terms appearing in parenthesis after each term.
Circular Optics (radially symmetric optics):
Transmissive optics showing radial symmetry.
Efficiency:
A ratio of the monoenergetic x-ray flux focused into first order to that incident on the active area of the optic. In general, the higher the efficiency, the better the optic.
Ion Beam Thinning. (ion beam polishing, ion milling):
A technique that uses a small ion gun with accelerating voltages typically less than 5 keV to remove material from a solid surface on the order of tens of Angstroms per second.
Lens:
A generic term referring to any transmissive focussing optic, including zone plates, phase plates, and gradient index phase plates.
Gradient Index Optic (blazed optic, gradient index zone plate):
A transmissive focussing optic whose index of refraction is a function of the distance across a zone. The zone boundaries are defined slightly different from that shown under the "zone plate equation", described hereinafter, to account for the phase shifting properties of a gradient index optic. A gradient index optic can be designed to be inherently more effective (approaching 100%) than a phase plate made of the same materials.
Phase Plate (phase zone plate):
A special case of the zone plate where the two materials are chosen such that when the final optic thickness is an odd integral multiple of pi-shifting-lengths, a local increase in focused intensity (and efficiency) is achieved. The maximum theoretical efficiency for a phase plate is 40%.
Sputter-Sliced Technique:
A fabrication method for making transmissive x-ray optics first by alternate sputter deposition of two materials onto flat or figured substrates or onto rotating wires. The optic is sliced normal to the film growth direction and initially thinned using standard metallographic techniques.
Transmissive Optic:
A generic class of optic where the incident wavefront is separated from the exiting wavefront by the optic. All transmissive optics discussed in the following description of the present invention will be focussing optics. This is in contrast to a reflective optic, where both wavefronts are on the same side of the optic.
Zone:
A radial distance in a transmissive optic whose boundaries correspond to a previously determined amount of phase shift between the face of the optic and the focus. The zone may be homogeneous, as in the case of a zone plate or phase plate, or vary in optical properties, as in the case of a gradient index optic.
Zone Plates (fresnel zone plate, Soret type zone plate):
A type of transmissive optic, characterized by alternating transparent and opaque zones, with the zone boundaries specifically placed to produce constructive interference at a desired focal point. Without any qualifiers, a zone plate relies solely on amplitude modulation; the maximum theoretical efficiency is 10%.
8 keV:
In the following description we refer to the Cu k-alpha radiation at 8.04 keV as "8 keV radiation".
Zone Plate Equation:
The relationship between the nth zone boundary location, r.sub.n, x-ray wavelength, .lambda., and focal length, f, that is used for both zone plate and phase plate designs: r.sub.n.sup.2 =n.lambda.f.
Gradient Index Equation:
A gradient index optic has the zone boundaries placed according to EQU r.sub.n.sup.2 =2.lambda.f.
Research and development has been directed to focussing electromagnetic radiation lying outside the visible spectrum, especially in the region of x-rays, for use in instruments, such as spectroscopes, microscopes, as well as for use in synchrotrons, x-ray lithography systems, x-ray mirrors, etc. These prior efforts are exemplified by U.S. Pat. No. 2,679,474 issued May 25, 1954 to W. S. Pajes, directed to a process of making optical zone plates for focusing x-ray energy; U.S. Pat. No. 3,927,319 issued Dec. 16, 1975 to D. B. Wittry, directed to a stepped curved crystal for an x-ray crystal spectrometer; U.S. Pat. No. 4,084,089 issued Apr. 11, 1978 to W. P. Zingaro et al., directed to an x-ray diffraction crystal for the analysis of x-rays having a wave-length of 50 Angstroms or greater; U.S. Pat. No. 4,242,588 issued Dec. 30, 1980 to J. K. Silk et al., directed to an x-ray lithography system for the production of micro-electronic circuits; U.S. Pat. No. 4, 684,565 issued Aug. 4, 1987 to B. Abeles et al., directed to an x-ray mirror including repeated multi-layered materials; and U.S. Pat. No. 4,469,933 issued Sep. 15, 1987 to J. E. Keem et al., directed to x-ray dispersive and reflective structures.
Various techniques have been developed for the fabrication of x-ray optics or lenses, as well as other components for focusing and/or utilizing x-ray energy. Among these fabrication techniques is the sputtered-sliced technique defined above using alternate sputter deposition of two materials on substrates or rotating wires, whereafter the coated substrate or wire is sliced to a desired thickness and then thinned by metallographic techniques. It has been demonstrated that the sputtered-sliced technique can be used to fabricate both linear and radially symmetric phase shifting optics for 8 keV x-rays. The following articles illustrate prior work directed to lenses for 8 keV x-rays: R. M. Bionta, "Transmission gratings that diffract 8 keV x-rays", Appl. Phys. Lett. 51 725 (1987); R. M. Bionta et al., "Sputtered-sliced linear zone plates for 8 keV x-rays", UCRL-97118 dated Oct. 1987, International Symposium on X-Ray Microscopy, Aug. 31-Sep. 4, 1987, published in X-Ray Microscopy, D. Sayre et al. editors, Springer-Verlag, New York (1987); R .M. Bionta, "Transmissive Optics for High-Energy X-Rays", Energy and Technology Review, Lawrence Livermore National Laboratory, July-August 1988, pp. 86-87; R. M. Bionta et al., "Sputtered-Sliced Multilayers:Zone plates and transmission gratings for 8 keV x-rays", SPIE, 1988; R. M. Bionta et al., "8 keV X-Ray Zone Plates", X-Ray/EUV Optics for Astronomy and Microscopy", R. B. Hoover editor, SPIE Vol. 1160, 12 (1989); R. M. Bionta et al., "Tabletop x-ray microscope using 8 keV zone plates", Opt. Engrg. 29(6) (1990),576; and R. M. Bionta et al., "Hard-X-Ray Lenses", Energy & Technology Review, LLNL, September-October 1991, pp. 8-14.
It has been determined that the high aspect ratio necessary for transmissive optics for hard x-rays (greater than about 4 keV in energy) can readily be achieved by this technique. Also, an amplitude modulating transmissive optic using the sputtered-sliced technique has been demonstrated. In addition, it has been indicated that both phase shifting and blazed-phase shifting optics could be made using this technique, although no fabrication process has been published.
In view of experience in fabricating the 8 keV optics, it was found that the necessary final thicknesses and tolerances cannot be achieved with standard metallographic procedures, particularly for soft x-ray optics in the 0.5 to 4.0 keV region. There is a need for transmissive x-ray optics capable of efficient operation in the lower or soft x-ray energy levels.
This need has been fulfilled by the present invention which involves a process for fabricating transmissive gradient index x-ray optics for the 0.5-4.0 keV energy range, thereby expanding the state of the art in both the fabrication of sputtered-sliced optics and in soft x-ray optics by a unique polishing and testing sequence.